The present invention relates to closure devices for lead-acid batteries. More particularly, it relates to those that incorporate activated carbon within a filter or catalytic device for the removal of poisons from the battery discharge gases.
The electrode grids of lead-acid electrochemical cells are produced from lead alloys that contain antimony or arsenic. Under certain conditions during use of the lead-acid battery, antimony hydride and arsenic hydride gases are produced and discharged. In addition to these poisonous gases, hydrogen and oxygen gas are also emitted during operation of the lead-acid electrochemical cell. As an illustration of a poisonous effect of SbH.sub.3 and AsH.sub.3 on the respiratory organs, a typical maximum concentration permitted in a work place is about 0.05 ppm for SbH.sub.3 and about 0.1 ppm for AsH.sub.3.
In a lead-acid battery grid that includes 1.6% by weight antimony in the lead alloy, a gas discharge can contain about 4 mg SbH.sub.3 per 100 liters of gas. For such a discharge, at least 100 fold air dilution is required to reduce this antimony content to a safe level. The extent of this problem is illustrated in that a typical submarine employing lead-acid storage batteries may develop about 1500 liters per hour gas discharge and in such an environment it is always possible that the maximum admissible concentration of these poisonous gases may be exceeded.
It is possible to catalytically decompose SbH.sub.3 and AsH.sub.3 by using CuO or PbO.sub.2. A process of this type as shown in Canadian Pat. No. 1024589. It has also been found that these poisonous hydrogen compounds can be removed through use of an activated carbon filter.
A filter plug of activated carbon may be damaged or destroyed by explosions that can occur within the head space of the lead-acid secondary cell from the accumulation of hydrogen and oxygen gases. In addition, the activated carbon can lose its effectiveness if it becomes wetted with water. Even if the activated carbon is made hydrophobic, the acid droplets which escape from the electrochemical cell or storage battery will during the course of time destroy the hydrophobic coating.
One other representative publication in this field is U.S. Pat. No. 4,084,039 entitled "Gas Venting Battery Plug" to Helmut Laig-Horstebrock. The device disclosed in this patent includes a nozzle directed toward an impingement plate for removing fine droplets of acid from the discharge gas flow. A return passage including a porous and wettable material extends into the storage battery for return of condensed liquid droplets but with sufficient hydrostatic head to prevent bypassing of gas discharge. However, a device of this type is without protection from explosions that may result in the discharge gas.